Ph.D Thesis

Ph.D StudentBordo Eliyahu
SubjectBi-Elliptical High-Harmonic Spectroscopy
DepartmentDepartment of Physics
Supervisor PROF. Oren Cohen
Full Thesis textFull thesis text - English Version


High harmonic generation (HHG) is an extreme nonlinear optical process where many visible or infrared photons are converted into a single high-energy photon in the extreme ultraviolet (XUV) and x-ray spectral regions. A significant application of HHG is high-harmonic spectroscopy (HHS), which employs direct and in-situ measurements of the HHG radiation for exploring processes within the nonlinear medium, possibly at attosecond temporal resolution.  HHS can be performed by measuring the HHG power spectrum and phase, characterizing the emitted attosecond pulses in the time-domain, and/or measuring the harmonic polarization. Polarization high-harmonic spectroscopy is becoming a promising method for exploring chiral quantities, also stimulated by development of techniques for generation of high-order harmonics with tunable, and in particular highly helical polarization. In this context, direct mapping (preferably analytical formula) between the harmonic polarization and time-domain description of the HHG process would be very useful.

This thesis includes two works. In the first work, I discovered a semi-analytical mapping between the ellipticities of the high harmonics and time-domain picture of the generation process in a new method for generation of bright helically polarized high harmonics. We investigate the underlying physical mechanism that governs this process, and formulate an analytical formula that links between the polarization properties of the harmonic radiation to various properties of the attosecond pulse trains (APTs) that are emitted in the HHG interaction. We explore this mapping theoretically (analytically and numerically) and experimentally. Utilizing this new mapping, we reconstruct the properties of the experimentally generated atto-pulse trains (including relative emission directions, relative delays between interlocked APTs and the polarization ellipticities of the APTs).

In the second work, high harmonic generation from noble gases is analyzed over the broad range of pump ellipticities of bi-elliptical laser field (fundamental and its second harmonic, where both beams are elliptical with perpendicular ellipses’ major axes and opposite helicity). My colleagues and I discovered that the generation process exhibits a relatively high sensitivity to varying the widths of the participating valence orbital and effective potential, while preserving the ionization potential. We identify the responsible physical mechanisms for this enhanced sensitivity through theoretical investigations. We demonstrate it experimentally by measuring significant disparities in the bi-elliptical HHG spectrograms of atomic Ar and Kr, which have relatively similar ionization potentials, but substantially different -shell widths and effective potentials.

I believe that the findings presented in this work will open new opportunities in high harmonic spectroscopy. The ellipticity-resolved spectroscopic technique may be utilized for time-resolved experiments of attosecond charge flow in molecules. The bi-elliptical scheme could be used for ultrafast spectroscopy of various quantities, such as the magnitude of coulomb-corrections in strong-field processes, the shape and width of the atomic orbitals, and the effective potential structure of the ionized target.